The present invention relates to a reforming reactor with three serially arranged reactor steps which are each charged with a catalyst pellet fill. Reactors of this type may be used to produce hydrogen from the water vapor reforming or partial oxidation of methanol. In a fuel-cell operated motor vehicle, such a reactor can be used to feed the fuel-cells with hydrogen.
This application claims the priority of German Patent Application No. 1 962 44 33.1-41, filed in Germany on Jun. 19, 1996, the disclosure of which is herein expressly incorporated by reference.
U.S. Pat. No. 5,248,566 (hereinafter the '566 patent) describes a reforming reactor which is suitable for use in fuelcell operated motor vehicles. In the reforming reactor disclosed in the '566 patent, the inlet-side reactor step is charged with an appropriate catalyst material designed for the partial oxidation of methanol, whereas the center reactor step is charged with another catalyst material suitable for the water vapor reforming of methanol. The outlet-side reactor step is designed to convert carbon monoxide into carbon dioxide. The heat required to implement the endothermal water vapor reforming reaction in the center reactor step is generated by the inlet-side reactor step by the exothermal partial oxidation of methanol and is charged with the reaction gas into the center reactor step.
In U.S. Pat. No. 5,401,589 (hereinafter the '589 patent), a three-step reforming reactor for the water vapor reforming of methanol is disclosed, wherein the reforming reactor has a reaction space which is filled with a continuous catalyst pellet fill and which is divided into three reactor steps arranged one below the other. According to the '589 patent, each reactor step is maintained at a different temperature. More specifically, the inlet-side reactor step arranged on top is maintained at approximately 300.degree. C., the center reactor step is maintained at approximately 275.degree. C., and the outlet-side reactor step is maintained at approximately 225.degree. C. The three reactor steps are heated with a heat conductive separating wall such that the reactor steps are in thermal contact with a common heating space through different contact surfaces.
U.S. Pat. No. 3,522,019 (hereinafter the '019 patent) discloses a reforming reactor with three serially arranged reactor steps which are separate from one another and which are each charged with a suitable catalyst fill. In the '019 patent, an inlet-side reactor step acts as a preliminary reactor and is heated to a temperature of approximately 370.degree. C. to 650.degree. C. The preliminary reactor has a heat exchange conduit which feeds hot reaction gas from the center reactor step into the preliminary reactor to thereby heat the corresponding reaction space. Another-heat exchange conduit extends from a burner to the center reactor step and heats the corresponding reaction space. An outlet-side reactor step acts as a shift conversion reactor for the exothermal conversion of carbon monoxide to carbon dioxide, wherein the shift conversion reactor heats a hydrogen purifier which has a membrane which is selectively permeable to hydrogen.
Published Japanese Patent Application JP 0 634 54 08 A discloses a reforming reactor with a stacked layer construction wherein a hydrogen discharge space is flanked on each side parallel to the stacking direction by a hydrogen permeable membrane whose other side is adjacent to one reforming catalyst reaction layer respectively. Raw hydrocarbon material and vapor are fed to the reforming catalyst reaction layer-perpendicular to the stacking direction to carry out the desired reforming reaction. On the side opposite the hydrogen discharge space, the reforming reaction layers are each adjacent to one combustion catalyst layer respectively, to which hydrocarbon fuel and air are fed perpendicular to the stacking direction. The resulting exothermal oxidation reaction heats the respective adjoining reforming reaction layers.
An object of the present invention is to provide a three-step reforming reactor which can be implemented at relatively low cost and which is particularly suitable for mobile applications.
This and other objects are achieved according to the present invention by a reforming reactor, particularly for the water vapor reforming of methanol, comprising three serially arranged reactor steps, i.e. an inlet-side reactor step, a center reactor step and an outlet-side reactor step, each of which is charged with a catalyst pellet fill, wherein the center reactor step is heated at a temperature which is suitable for carrying out the reforming reaction, while the two other reactor steps remain unheated.
According to the present invention, the inlet-side reactor step is unheated but its corresponding reaction space, as well as the reaction space of the other two reactor steps, are charged with a suitable catalyst pellet fill. The inlet-side reactor step is therefore used as a filter to filter out floating particles as well as water drops or methanol drops, which prevents damage to the catalyst material in the center reactor step. In addition, the inlet-side reactor step carries out a partial conversion of the reforming reaction, which relieves the center reactor step. Furthermore, temperature fluctuations of the educt gas caused by load changes, which are common in mobile uses such as in motor vehicles, are dampened by the heat capacity of the inlet-side reactor step and by cooling the entering hot gas flow because of the partial endothermal conversion reaction.
As a result of the unheated outlet-side reactor step, it is possible to convert carbon monoxide to carbon dioxide in order to minimize the carbon monoxide content, which is particularly desirable for subsequent use of the reformed gas in a fuel-cell arrangement. Additionally, as a result of the heat capacity of the outlet-side reactor step, temperature fluctuations in the center reactor step occurring because of load changes, are further dampened. As a result, only comparatively slight fluctuations of the carbon monoxide content and the outlet gas temperature occurs during load changes.
The actual reforming reaction is carried out in the center reactor step and, because of the carbon monoxide conversion occurring in the outlet-side reactor step, can be operated at higher temperatures and with larger temperature fluctuations.
Therefore, for a given reforming capacity requirement, the reforming reactor of the present invention permits a simple and compact reactor construction, wherein the reactor is relatively insensitive to temperature fluctuations occurring as a result of load changes.
In a preferred embodiment, the three reactor steps of the reforming reactor are formed by superimposed sections of a reaction space which is filled with a continuous catalyst pellet fill, wherein only the center reactor step of the reaction space is in thermal contact with a heating space.
The continuous catalyst pellet fill of the reaction space permits automatic filling of the center reactor step with material from the inlet-side reactor step, which is situated above it, when the catalyst pellet fill exhibits settling caused by jolts occurring in mobile applications in motor vehicles for example. In this preferred embodiment, diagonal installation of the reforming reactor in the vehicle is also possible without disturbing the automatic filling function.
In certain preferred embodiments, the center reactor step, which carries out the reforming reaction, is constructed as a reactor tube bundle wherein tubes extend from the top inlet-side reactor step, which operates as a distributor space, through an intermediate heating space to the bottom outlet-side reactor step, which operates as a collection space.
In certain other preferred embodiments, the center reactor step has a heating space which is constructed as a heating tube bundle with tubes extending transversely through the center reactor step.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.